The aim of our study in 1981-82 is to compare on Blacks two methods for estimating body fat. One method involved estimating the amount of K-40 in the body. Knowing the ratio of K-40 to K one obtains an estimate of total potassium in the body. Nearly all the potassium of the body is found in a constant proportion in the protein of the body. Thus, it becomes possible to calculate the total protein of the body. Protein is the chief component of the body other than body fat so it is assumed the difference between body weight and protein or lean body mass is fat. The other method is based on underwater weighing and the assumption that the density of lean body mass is l.l and of body fat 0.93. Body weight in air and body weight in water by the Archimedes principle makes it possible to calculate body volume but a correction must be made for air in the lungs. Even after a maximal expiration more than one liter of air remains in the lungs. This "residual volume" is determined and subtracted from the calculated value for body volume to obtain the air-free body volume. The density of the body is then calculated and then body fat by a formula based on the density values stated above. No such comparison has been made on Black males or females. On Whites these two methods agree well except in old age. Both methods of calculation are empirical; possibly those empirical formulas will need a correction for Blacks.